Equine influenza virus has been recognized as a major respiratory pathogen in horses since about 1956. Disease symptoms caused by equine influenza virus can be severe, and are often followed by secondary bacterial infections. Two subtypes of equine influenza virus are recognized, namely subtype-1, the prototype being A/Equine/Prague/1/56 (H7N7), and subtype-2, the prototype being A/Equine/Miami/1/63 (H3N8). Presently, the predominant virus subtype is subtype-2, which has further diverged among Eurasian and North American isolates in recent years.
The currently licensed vaccine for equine influenza is an inactivated (killed) virus vaccine. This vaccine provides minimal, if any, protection for horses, and can produce undesirable side effects, for example, inflammatory reactions at the site of injection. See, e.g., Mumford, 1987, Equine Infectious Disease IV, 207-217, and Mumford, et al., 1993, Vaccine 11, 1172-1174. Furthermore, current modalities cannot be used in young foals, because they cannot overcome maternal immunity, and can induce tolerance in a younger animal. Based on the severity of disease, there remains a need for safe, effective therapeutic compositions to protect horses against equine influenza disease.
Production of therapeutic compositions comprising cold-adapted human influenza viruses is described, for example, in Maassab, et al., 1960, Nature 7, 612-614, and Maassab, et al., 1969, J. Immunol. 102, 728-732. Furthermore, these researchers noted that cold-adapted human influenza viruses, i.e., viruses that have been adapted to grow at lower than normal temperatures, tend to have a phenotype wherein the virus is temperature sensitive; that is, the virus does not grow well at certain higher, non-permissive temperatures at which the wild-type virus will grow and replicate. Various cold-adapted human influenza A viruses, produced by reassortment with existing cold-adapted human influenza A viruses, have been shown to elicit good immune responses in vaccinated individuals, and certain live attenuated cold-adapted reassortant human influenza A viruses have proven to protect humans against challenge with wild-type virus. See, e.g., Clements, et al., 1986, J. Clin. Microbiol. 23, 73-76. In U.S. Pat. No. 5,149,531, by Youngner, et al., issued Sep. 22, 1992, the inventors of the present invention further demonstrated that certain reassortant cold-adapted human influenza A viruses also possess a dominant interference phenotype, i.e., they inhibit the growth of their corresponding parental wild-type strain, as well as heterologous influenza A viruses. U.S. Pat. No. 4,683,137, by Coggins et al., issued Jul. 28, 1987, and U.S. Pat. No. 4,693,893, by Campbell, issued Sep. 15, 1987, disclose attenuated therapeutic compositions produced by reassortment of wild-type equine influenza viruses with attenuated, cold-adapted human influenza A viruses. Although these therapeutic compositions appear to be generally safe and effective in horses, they pose a significant danger of introducing into the environment a virus containing both human and equine influenza genes.